The instant invention relates generally to electric motors and more particularly to an improved rotor laminate disc design for an alternating current (AC) induction motor. Large electric motors, for example AC induction motors, employ rotors having a plurality of longitudinal conductive elements, often comprised of copper, copper alloys, or aluminum alloys, radially spaced from a central axis thereof, that operate to enhance the formation of an electromagnetic field therein.
The rotor is comprised of a plurality of thin, coated, electrical steel discs, each having a central aperture therein through which a rotor shaft is pressed. The discs further have a plurality of radially spaced slots therein for the acceptance of longitudinal conductive elements. The discs are stacked and welded or pressed together to form a generally right circular cylindrical assembly, having a central longitudinal aperture therethrough, and a plurality of longitudinal radially spaced slots therein that define longitudinal channels located proximate the periphery of the assembly.
The central aperture of the cylindrical disc assembly is then pressed over a rotor shaft. The conductive elements (hereinafter xe2x80x9cbarsxe2x80x9d) are then driven into the longitudinal channels to complete the rotor assembly.
Known-in-the-art motor designs operate most efficiently when the longitudinal conductive bars disposed in the rotor slots are positioned in very close proximity to a stator disposed radially outwardly of said rotor. Stated another way, the slots in each disc are ideally disposed proximate the circumference of the disc, thereby allowing the longitudinal elements to be located as close as possible to the radially outer surface of the rotor. This arrangement of slots and bars therein provides for reduced incidences of magnetic xe2x80x9cshortingxe2x80x9d that is detrimental to motor efficiency, but requires the portion of each disc radially outward of each slot (the slot xe2x80x9cbridgexe2x80x9d) to bear a large force as the rotor spins.
Prior art rotor designs employing traditional laminated disc slots suffer from a number of disadvantages. Initially, a large number of laminated discs must be stacked to form the rotor assembly, requiring both precise machining of slots in each disc and very precise alignment of each disc in the stack to provide accurate alignment of each slot therein with the slots in the adjacent disc(s). However, imprecise assembly of the laminate stacks in conjunction with variations in slot locations in each disc caused by machining tolerances provide for both radial and tangential offset in the channels formed by the alignment of the slots in each laminate layer. This radial and tangential offset in the corresponding disc slots causes great difficulty in rotor assembly because the conductive bars must be driven into the slot channels using a drift. This technique requires great force to be used to drive the bars completely into the channels, often requiring the use of air hammers.
In addition to making installation of the longitudinal bars difficult, the aforementioned radial and tangential offset causes difficulty in balancing the rotor at high revolutions. In many AC induction motor applications, for example an AC motor driven by a vector drive, it is desirable that the rotor spin at many different angular velocities. This requires a very precisely balanced rotor assembly to avoid excessive vibration, overheating, and damage of sensitive rotor and stator components due to interference. Rotor assemblies having poorly aligned slots or slots having inconsistent shapes are difficult or impossible to balance, and therefore are not suitable for high revolution applications.
Additionally, with conventional laminate slot shapes wherein some offset has occurred during assembly, the longitudinal bars may not contact the radially outward surface of each slot in the assembly. As the rotor spins, the centrifugal force generated by the bars on the slots in contact with the bars is much greater than those not in contact with the bars. In some cases, the bridge over a lamination slot may fail or yield as a result of the excess force placed thereon by the bar.
Furthermore, conventional laminate disc designs employ a relatively large bridge to provide sufficient strength to counterbalance the centrifugal force generated by the bars. However, this relatively large bridge inhibits the propagation of the electromagnetic field in the rotor, thereby decreasing the efficiency of the motor.
The aforementioned problems are solved by the electric motor rotor laminate disc of the present invention that provides accurate placement of the longitudinal conductive elements (bars) by utilizing a novel laminate disc design that enhances the accuracy of rotor assembly thereby allowing for better rotor balance over a wider rpm range.
Additionally, the laminate disc design of the instant invention provides a plurality of radially spaced slots therein that have a pair of radiused sections on the radially outer edge thereof allowing the longitudinal bars to contact the outer edge of each slot and obviating failure or yield of the slot bridges caused by excessive centrifugal force.
The instant invention employs a reduced bridge center depth allowing for enhanced electromagnetic field strength through the rotor. This feature of the instant invention greatly enhances motor efficiency.
Therefore, one object of the instant invention is a motor rotor that remains balanced over wide RPM ranges and in high RPM applications.
Another object of the instant invention is a rotor utilizing a plurality of novel laminate discs for enhanced accuracy and ease of assembly.
Another object of the instant invention is a laminate disc having a novel slot design to provide enhanced strength and more efficient motor operation.
Additional objects, features, and advantages of the present invention will become apparent from the subsequent detailed description, taken in conjunction with the accompanying drawing figures.